The present invention relates to a method of and an apparatus for, correcting color reproduction implemented by a color display device such as a color fluorescent tube or a color filter, and more particularly to the method of and apparatus for, correcting a color reproduction distortion appearing about all hues.
In a today's color TV receiver, the fluorescent tube for three primary colors (red, blue, green) provides a different xy chromaticity value from that defined according to the NTSC system. As such, the color-reproducing area of the color TV receiver is narrower than that of the NTSC fluorescent tube. This results in a distortion of the color-reproducing characteristic of the color TV receiver.
This is because the color-reproducing area of the NTSC fluorescent tube has been selectively set to the widest area of the NTSC fluorescent tube obtained at that time. Today, this setting is disadvantageous with respect to light of brightness (luminous efficiency) and persistence (short persistence).
FIG. 1 shows a distortion brought about when the fluorescent tube for three primary colors (red, blue, green) of the current color TV receiver provides a different xy chromaticity value from the NTSC fluorescent tube. In FIG. 1, a dotted line indicated by 1 denotes a color-reproducing area reproduced by the NTSC fluorescent tube. A real line indicated by 2 denotes a color-reproducing area reproduced by the current color TV receiver. Circles 3, 4, 5, 6, 7, 8, and 9, respectively, denote colors reproduced in the color-reproducing area 1 of the NTSC fluorescent tube, concretely, green, olive color, flesh color, red, pink, and blue. Dots: denote the similar reproduced colors in the color-reproducing area 2 of the current color TV receiver. An arrow indicates how a location of a reproduced color in the color-reproducing area 1 is changed into another location in the color-reproducing area 2 as a result of a distortion of the reproduced color. The tip of the arrow indicates that a distorted color is reproduced at the location (dot:) in the color-reproducing area 2. Double circles 4 and 9 denote the colors which are subject to no distortion.
As shown in FIG. 1, the color-reproducing area 1 produced by the NTSC fluorescent tube is different from the area 2 produced by the current color TV receiver. The difference brings about a distortion of a color-reproducing characteristic of the color TV receiver which causes a shift of almost all of the reproduced colors toward an axis 10 connected between the olive color 4 and the blue 6. This color shift results in such a color distortion as compression of the green 3 and the flesh color 5 toward the olive color 4. That is, the color reproduction distortion, in principle, takes place in a regular manner. If there is provided a line of axis having a certain hue (line of axis in FIG. 1), the colors actually reproduced by the color TV receiver are distorted to shift toward the axis 10 further than the color-reproduced location defined by the NTSC fluorescent tube. The shift of the colors makes the color reproduction of the color TV receiver inferior.
As an example of solving the shortcoming, a technique has been published in Japanese Patent Publication No. 57-23478. Hereafter, this prior art will be described as referring to Japanese Patent Publication No. 57-23478.
As described above, the reproduced color is distorted to shift toward a certain axis. This prior art is designed to focus on this negative feature. That is, it is arranged to magnify the color change toward both sides of the axis line for correcting the distortion, thereby enhancing the color reproduction.
FIG. 2 shows a uniform chromaticity scale (referred to as a UCS) indicating a color-reproducing characteristic provided as a result of correcting a color reproduction distortion in an example of a color reproduction correcting apparatus. In FIG. 2, 10 indicates an axis line of an olive color toward which the color reproduction distortion is compressed from both sides. That is, the color change is magnified in the direction indicated by arrows 11 so that the compression of each color toward the axis line 10 is corrected for overcoming the color reproduction distortion.
The method for this function will be described with reference to FIG. 3, which is a diagram showing a signal processing characteristic for providing the color reproducing characteristic shown in FIG. 2. In the diagram shown in FIG. 3, an axis of abscissa denotes a phase of an input chroma signal in the color TV receiver and an axis of ordinate denotes a phase of a corrected chromatic signal. If not corrected, the phase relation between these chroma signals is shown by a broken line. If corrected by the prior art, the phase relation between these chroma signals is shown by a real line. This correction causes the color to be shifted in the direction indicated by arrows 11 shown in FIG. 2.
FIG. 4 is a block diagram showing a conventional color reproduction correcting apparatus for correcting a color reproduction distortion according to a d. c. control type hue adjusting system. In FIG. 4, a numeral 12 denotes a bandwidth amplifier. A numeral 13 denotes a reference color carrier oscillator. A numeral 14 denotes a phase-shift circuit. A numeral 15 denotes a 90-degree phase advancer. A numeral 16 denotes a hue adjustor. A numeral 17 denotes a limiter. Numerals 18 and 19 are phase detectors. A numeral 20 denotes a clipper. A numeral 21 denotes a multiplier. A numeral 22 denotes a d. c. power source for adjusting a hue. A numeral 23 denotes an adding circuit. A numeral 30 denotes a color demodulating circuit.
In FIG. 4, a chroma signal contained in the received color video signal is band-limited by a band-pass filter 12. The resulting chroma signal is supplied to a color demodulating circuit 30 and the phase detectors 18, 19 through a limiter 17. A burst signal, picked up by the band-pass amplifier 12, is supplied to the reference color carrier oscillator 13 in which a reference color carrier synchronous in phase with the burst signal can be obtained. The reference color carrier is sent to the phase-shift circuit 14 in which it is phase-shifted. Then, the phase-shifted color carrier is directly supplied to the phase detector 18 as well as the 90-degree phase advancer 15. The phase-advanced color carrier is sent to the phase detector 19. For example, assuming that the phase of the output reference color carrier corresponds to an olive color by properly selecting the characteristics of the phase advancer 14, that is, the phase of the output reference color carrier is located 5 degrees against the phase of the burst signal of the input chroma signal, the phase detector 18 operates as a phase detector for olive color and the phase detector 19 operates as a phase detector for an axis perpendicular to the olive color signal.
FIGS. 5A to 5E are diagrams for describing the operation of the conventional color reproduction correcting apparatus. Assuming that the phase of an olive color signal is a reference phase and the phase of a chrominance signal against the reference phase is .theta., the voltage level of an output signal V1 of the phase detector 18 changes like a curve V1 against the phase .theta. as shown in FIG. 5A. The voltage level of an output signal V2 of the phase detector 19 changes like a curve V2 as shown in FIG. 5B. The output signal V1 of the phase detector 18 is clipped at a predetermined clipping level by the clipper 20. The resulting signal V3 provides a voltage characteristic changing like a curve V3 against the phase .theta. as shown in FIG. 5C. As such, the output signal of the clipper 20 is properly adjusted so as to obtain the amendment 11 of FIG. 2 around the olive color. As an example, herein, the clipping level of the clipper 20 is selected in the range of -60.degree. to +60.degree. . The output signal V3 of the clipper 20 is multiplied by the output signal V of the phase detector 19 in the multiplier 21. The multiplier 21 provides an output signal V4 having a voltage level changing like a curve V4 as shown in FIG. 5(D). The output signal (voltage) of the multiplier 21 is added to a d. c. voltage Vd from the d. c. power source 22 in the adding circuit 23. The added voltage is supplied to the hue adjustor 16 as a control voltage.
The hue adjustor 16 serves to change the phase of the reference color carrier sent from the reference color carrier oscillator 13 according to the control voltage sent from the adding circuit 23. The change of the phase is carried out by adding a chrominance sub carrier and its 90-degree phase-shifted carrier to each other at a rate defined according to a control signal. The change of the color sub carrier performed by the hue adjustor 16 is controlled by the control voltage sent from the adding circuit 23, that is, the output voltage Vd of the d. c. power source 22 and the output voltage V4 of the multiplier 21. Assume that the axis of ordinate indicates the output voltage Vd of the d. c. power source 22 and the axis of abscissa indicates the phase .DELTA..theta. of the color sub carrier of the hue adjustor 16 against the reference color carrier. This phase .DELTA..theta. changes like a characteristic curve V5 of FIG. 5 with respect to the output voltage Vd of the d. c. power source 22. The control voltage for the hue adjustor 16 is an addition of the d. c. voltage Vd output from the d. c. power source to the voltage V4 output from the multiplier 21 having the characteristic as shown in FIG. 5D. Assuming that the output voltage V4 of the multiplier 21 changes in the range of -e to +e in FIG. 5D, the output voltage of the adding circuit 23 changes in the range of .+-.e with respect to the predetermined d. c. voltage Vd applied by the d. c. power source 22 in FIG. 5E. Hence, if the phase of the chroma signal stays out of the range of -60.degree. to +60.degree. , the phase .DELTA..theta. against the d. c. voltage Vd is .DELTA..theta..sub.O. If, however, the phase of the chroma signal stays in the range, the phase .DELTA..theta. changes in the range of .DELTA..theta..sub.1 to .DELTA..theta..sub.2 (.DELTA..theta..sub.1 &lt;.DELTA..theta..sub.2 &lt;.DELTA..theta..sub.3). Such a reference color carrier is fed from the hue adjustor 16 to the color demodulating circuit 30. In the color demodulating circuit 30, the chrominance signal sent from the band-pass amplifier 12 is demodulated. This results in demodulating a chrominance signal having a characteristic of magnifying the change of a hue around the olive color as described in FIG. 3 and correcting a color reproduction distortion as described in FIG. 2.
The foregoing prior art is effective in correcting the colors near the axis line 10 shown in FIG. 2 but has difficulty in correcting all the colors. This shortcoming will be described later.
In the case of a characteristic curve shown in FIG. 3, the color reproduction distortion about the green 3 or the flesh color 5 is improved as shown in FIG. 2. However, with respect to the red 6, the cyan 7, and the pink 8 as shown in FIG. 1, those colors are not included in the amending range of the characteristic curve as shown in FIG. 3. Hence, the color reproduction distortion about these colors is not improved.
FIG. 6 is a UCS diagram showing an example of the method for correcting color reproduction assumed from the conventional color reproduction correcting apparatus shown in FIG. 4.
FIG. 7 is a UCS diagram showing another example of the method for correcting color reproduction assumed from the conventional color reproduction correcting apparatus shown in FIG. 4.
Herein, consider the case where the correcting range of the characteristic curve shown in FIG. 3 is expanded. The corrected result is as shown in FIG. 6. In this case, the correction is executed to magnify the color change toward arrows 11. The red color 6 is shifted from the color-reproduced location 61 before correcting to the color-reproduced location 62 after correcting in the lower of the diagram. Likewise, the cyan color 7 is shifted from the color-reproduced location 71 before correcting to from the color-reproduced location 72 after correcting in the upper of the diagram. The pink color 8 is shifted from the color-reproduced location 81 before correcting to the color-reproduced location 82 after correcting in the lower of the diagram. That is, by magnifying the correcting range of the characteristic curve as shown in FIG. 3, no improvement of the color reproduction distortion about the red 6, the cyan 7, and the pink 8 is carried out.
In FIG. 3, the olive color 4 is defined as the center of correction. In addition to it, a blue color 9 is also defined as the other center of correction. That is, the centers of correction are defined. Considering the case where the correcting range stays from -90.degree. to +90.degree. , the corrected result is shown in FIG. 7. In this case, the correction is directed towards two ways, that is, an arrows 11 and arrows 110. By doing such correction, the color reproduction distortion about the pink 8 is improved according to a user's will, while the red color 6 is merely shifted from the color-reproduced location 61 before correcting to the location 63 after correcting in the lower of the diagram and the cyan color 7 is merely shifted from the color-reproduced location 71 before correcting to the location 73 after correcting in the upper of the diagram. It means that the color reproduction distortion about each of these colors is still disadvantageously present.
As described above, the foregoing prior art has difficulty in improving a color-reproducing characteristic about each of the colors in the color reproduction range. Further, to increase the number of colors to be improved, the prior art needs two or more center points of correction. This leads to disadvantageous magnification of a circuit scale according to the increase of the center points.